[moves] Implement GeneratedMove::ply()

Returns a copy of the generated move. This is simplier to use than ::into().

moves/src/generators.rs

@@ -30,6 +30,11 @@ impl GeneratedMove {
     pub fn target(&self) -> Square {
         self.ply.target_square()
     }
+
+    #[must_use]
+    pub fn ply(&self) -> Move {
+        self.ply
+    }
 }
 
 impl std::fmt::Display for GeneratedMove {